1. Field of the Invention
The present invention relates to a probing card for wafer testing and, more particularly, to a probing card for wafer testing having a plurality of probes arranged so as to correspond to and be in contact with a plurality of electrodes of a semiconductor device (semiconductor chip) fabricated on a semiconductor wafer.
2. Description of the Background Art
Generally, a checking equipment (hereinafter referred to as "wafer prober") is used for measuring electrical characteristics of a semiconductor device formed on a semiconductor wafer in a semiconductor integrated circuit (IC) manufacturing process. This wafer prober employs a probing plate formed by fixedly arranging probes in a pattern corresponding to that of the electrodes, i.e. bonding pads, of the semiconductor device on an insulting holding plate. FIG. 5 is a sectional view schematically showing a conventional probing plate for wafer testing incorporated into a wafer prober. FIG. 6 is an enlarged fragmentary perspective view showing slender electrical leads (hereinafter referred to as "probes") in contact with bonding pads on a wafer. A conventional LSI tester for wafer testing will be described with reference to these drawings.
A wafer 1 mounted with a plurality of semiconductor devices, such as LSIs (large-scale integrated circuits) is fixed on a wafer chuck 8. The wafer chuck 8 can be moved vertically and horizontally by a moving mechanism 9. A large number on the order of several hundreds of bonding pads 2 are formed on the upper surface of the wafer 1 for each semiconductor chip. Slender probes 3 are arranged and held on a probe holding plate 4 so as to correspond to the bonding pads 2, respectively. The probe holding plate is positioned above the wafer 1. The probes 3 are formed of tungsten, chromium or a tungsten-chromium alloy. The probe holding plate 4 is formed of an insulting material, such as glass or epoxy resin. A conductive pattern of metallic wiring lines 5 is formed on the probe holding plate 4. This conductive pattern 5 is formed of copper, for example. The probes 3 are connected electrically to the conductive pattern 5 by soldering or the like. The size of the contact area of the electrode pads 2 with which the tip of the probe 3 is brought into contact is in the range of about 50 to 100 .mu.m in diameter. The external diameter of the base of the probe 3 connected to the probe holding plate 4 is in the range of about 150 to 200.mu.m. A conductive pattern 5, which is the same as the conductive pattern 5 connected to the probes 3, is formed on the other surface of the probe holding plate 4 and is connected via through holes to the former conductive pattern 5. The conductive pattern 5 is in electrical contact with contact pins (hereinafter referred to as "pogo pins") 7 provided on the test head 6 of an LSI tester. The probe holding plate 4 is held fixedly on a plate holding member 12.
The testing procedure using the probing plate (or probing card) for testing the electrical characteristics of semiconductor devices on a wafer will be described hereinafter. The position of the wafer chuck 8 is adjusted by the moving mechanism 9 so that the bonding pads 2 of one or a plurality of semiconductor devices formed on the wafer 1 are brought into positions opposed respectively to the corresponding probe 3 fixed on the probe holding plate 4. The wafer chuck 8 is moved vertically so that the surfaces of the bonding pads 2 are in appropriate contact with the corresponding probes 3, respectively. Consequently, the semiconductor devices formed on the wafer 1 are connected electrically through the conductive patterns 5 and the pogo pins 7 of the test head to the LSI tester for signal transmission between the semiconductor devices on the wafer 1 and the LSI tester for testing the electrical characteristics of the semiconductor devices assembled on the wafer 1.
However, since the conventional probing plate for wafer testing employs probes, the diameter of the probes must be reduced according to a reduction in the size and pitch of the bonding pads formed on the wafer, which entails an increase in the contact resistance between the boding pads and the probes. Furthermore, difficulty in manufacturing the probes and in mounting the probes in the probe holding plate is enhanced as the diameter of the probes is reduced. That is, a reduction in the size of probes makes it difficult to accurately attach a plurality of minute probes to the probe holding plate so that the probes can be highly accurately positioned relative to a plurality of bonding pads formed on the wafer by precision processes.
Japanese Patent Laying-Open No. 63-263738 discloses a probing card with improved precision for positioning wafer testing probes to solve the foregoing problems. FIG. 7 is a fragmentary sectional view showing a probe portion of such a probing card. This probing card comprises an insulating base plate 21 carrying conductive patterns 22 formed on both surfaces thereof. These conductive patterns 22 are electrically interconnected through conductors 23 formed in through holes. A bump 24 is formed on each conductive pattern 22 on one surface of the base plate 21. The conductive patterns 22 are formed by etching copper foil applied to the base plate 21. The bumps 24 are formed of gold.
According to the above probing card, the bumps 24 are formed on extreme ends of the conductive patterns 22 to act as probes. Consequently, a high precision for positioning the probe portion may be secured by designing the conductive patterns 22 in accordance with positions of electrodes or bonding pads to be measured. However, the probe portion, because of the bump shape, makes a contact with each bonding pad in the form of a point contact. Such a contact between the bump 24 and the bonding pads cannot be a stable contact. When the bumps 24 are given in an increased height so that the bumps 24 alone will reliably come in contact with the surfaces of the bonding pads, the bumps 24 must extend across the conductive patterns 22 because of the shaping condition. Consequently, even if the conductive patterns 22 may be formed with niceties, it is difficult, because of the limitation imposed by the bump shape, to form the probe portion with desired minute spacing and high density.
Japanese Patent Laying-Open No. 64-39559 discloses a different probing card with improved precision for positioning wafer testing probes. FIG. 8 is a fragmentary sectional view showing a probe portion of such a probing card. This probing card comprises a transparent base plate 31 fixedly carrying probes 34 formed of a conductive elastic material for coming in contact with the bonding pads of each chip formed on a semiconductor wafer. The base plate 31 further carries conductive wiring layers 32 connected to the probes 34. On the conductive layers 32 are compressed gas filled layers 33 surrounded by partitions 35 formed of a transparent material. A high pressure gas filled into the layers 33 applies a pressure to the probes 34. This assures an excellent contact between the probes 34 and the bonding pads. That is, a uniform probe pressure is applied to the respective bonding pads.
According to the above probing card, however, the probes 34 are bump-shaped and are placed in through holes of the base plate 31 for connection to the conductive layers 32. Such a limitation to the shape of the probes is obstructive to a stable contact between the bonding pads and probes. This is because the contact between the bonding pads and probes takes the form of a point contact having a small contact area. In order for only the tip faces of the probes 34 to come in contact with the bonding pads reliably, the tip faces must be located remote from the surface of the base place 31. If this is realized by increasing the height of the probes 34, the probes 34 are inevitably formed to extend transversely on the surface of the base plate 31. This makes it difficult of form the probes 34 with desired minute spacing and high density. Besides, the probes 34 are connected to the conductive layers 32 via the through holes. The probes 34 are repeatedly pressurized for one measurement after another. It is therefore possible for the probes 34 shaped as shown in FIG. 8 to slip downwardly out of the through holes.